Antihistamines are able to disrupt and thereby prevent the immune response to an allergen but their long-term effects on the immune system have not been investigated. However, a recent investigation suggests that mice, sensitised to bee venom and subsequently given antihistamines reacted more violently to a second bee venom injection than the mice who had not been given antihistamines.

‘We believe that the antihistamines were doing more than disrupting the immediate immune reaction to the first venom dosage,’ says Pål Johansen at the University of Zurich where the work was done. ‘We think they were also keeping the immune system from getting used to that dosage.’ That means the mice on allergy medication had no chance to build up a tolerance to the allergen.

In a second part of the study, Johansen’s team desensitised the mice to the bee venom using immunotherapy — introducing tiny amounts of an allergenic substance to an allergic individual, slowly helping the immune system to tolerate the substance. Unlike antihistamines, immunotherapy can permanently change the way the body responds to a substance and cure allergies.

Mice that had originally been on antihistamines when sensitised with venom responded more poorly to the immunotherapy than the other mice. ‘This was really surprising because others have claimed that antihistamines would aid the process of immunotherapy, and that is simply not what we saw,’ says Johansen.

Johansen, P. et al. Clin. Exp. Allergy DOI 10.1111/j.1365-2222.2007.02904.x (2008)

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First published in March 2008

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